1. Field of the Invention
The present invention generally relates to a fixing device used in an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions, and an image forming apparatus including the fixing device.
2. Discussion of the Background
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction machines including at least two of these functions, include an image carrier on which an electrostatic latent image is formed, a developing unit to develop the latent image with toner, a transfer member, and a fixing device. The developed image (toner image) is transferred from the image carrier onto a sheet of recording media by the transfer member and then fixed on the sheet with heat and pressure by the fixing device.
Fixing devices that include an endless belt member, a fixed member that is fixed inside the belt member to slidingly contact an inner circumferential surface (hereinafter also “sliding surface”) of the belt member, and a rotary member against which the fixed member presses via the belt member are widely used. The toner image is fixed on the sheet when the sheet passes through a nip portion (hereinafter also “fixing nip”) where the belt member and the rotary member press against each other.
An on-demand fixing device whose warm-up time is relatively short is known. The on-demand fixing device includes an endless belt-line fixing film as a belt member, a pressure roller as a rotary member, and a glass-coated ceramic heater as a fixed member that is fixed inside the fixing film. The heater slidingly contacts an inner surface of the fixing belt that is formed with resin such as polyimide, or metal such as stainless steel or nickel and presses against the pressure roller via the fixing film, forming a fixing nip between the fixing film and the pressure roller. The heater heats the fixing film around the fixing nip, and the toner image is fixed on the sheet with heat and pressure when the sheet passes through the fixing nip, that is, when the sheet is conveyed between the belt member and the rotary member.
In such fixing devices, because the fixed member slidingly contacts the inner circumferential surface of the belt member for a relatively long time period, both the fixed member and the belt member experience wear.
For example, in the above-described known fixing device, the fixed member and the fixing belt can wear out over time because sliding resistance between the fixing belt and the fixed member is relatively large, and accordingly the operational life of the fixing device can be relatively short. Additionally, driving torque of the fixing device is relatively high due to the high sliding resistance between the belt member and the fixed member, which might cause the belt member to slip, disturbing a fixed image, which is a phenomenon so-called “image slip”. Relatively high driving torque of the fixing device can also damage a teeth of a driving gear used to drive the fixing device.
Therefore, several approaches as described below have been tried to enhance slidability between the belt member and the fixed member that slidingly contact each other in order to reduce wear on them.
For example, another known fixing device includes an endless pressure belt whose inner circumferential surface is formed with resin such as polyimide, a fixing roller serving as a rotary member, a heating member such as halogen lamp provided inside the fixing roller, and a pressure pad serving as a fixed member fixed inside the pressure belt to slidingly contact the inner circumferential surface of the pressure belt. Being urged by a spring toward the fixing nip, the pressure pad presses against the fixing roller via the pressure belt, forming a fixing nip between the pressure belt and the fixing roller.
In this known fixing device, glass cloth impregnated with PTFE (polytetrafluoroethylene) having a relatively low resistivity is provided on a surface (sliding surface) of the fixed member that slidingly contacts the inner circumferential surface of the pressure belt to enhance slidability between the pressure pad and the pressure belt.
Other known fixing devices use a pressure pad as a fixed member, and silicone oil is provided between the pressure pad and the pressure belt.
Yet other known fixing devices include an endless fixing belt, a heating member provided inside the fixing belt, a pressure roller, and a belt guide serving as a fixed member that slidingly contacts an inner circumferential surface of the fixing belt and presses against the fixing roller via the fixing belt. In this known fixing device, a surface layer (sliding layer) including PFA (perfluoro alkoxy), PTFE, and the like is provided on both a surface of the belt guide and the inner circumferential surface of an endless fixing belt that slidably contact each other.
However, the above-described approaches may be insufficient to reduce wear of the belt member and the fixed member. Although wear of the belt member and the fixed member can be reduced to a certain extent, it is difficult to keep a sufficient slidability between the belt member and the fixed member for a relatively long time period.
For example, in the above-described fixing device using the glass cloth impregnated with PTFE (low-resistivity sheet), if the fixing device is operated for a relatively long time period, PTFE of the glass cloth can wear over time, exposing the glass cloth. In such a case, the sliding resistance between the pressure belt and the pressure pad might increase rapidly.
Further, in the above-described fixing device using silicone oil (lubricant), it is difficult to maintain a sufficient amount of lubricant on the sliding surfaces of the fixed member and the belt member for a relatively long time period. If the fixing device is operated for a relatively long time period, the lubricant might be removed from the surfaces, increasing the sliding resistance between the belt member and the fixed member rapidly.
Similarly, in the above-described fixing devices in which the surface layer including PFA, PTFE, and the like is provided on the sliding surfaces of the fixed member and the belt member, and lubricant is applied between the sliding surfaces, if the sliding surfaces are smooth, the lubricant might be removed therefrom over time.
In view of the foregoing, a need has arisen to maintain lubricant between the fixed member and the belt member for a relatively long time period even when the fixed member slidingly contacts the inner circumferential surface of the belt member.